This invention relates to carbon monoxide (CO) detectors, and particularly to those which use an oxidation process and heat change measurement for sensing the amounts of carbon monoxide.
One of the important concerns relates to automotive exhaust measurements, wherein the gas flow is continuous. Sensitivity, quick response, and accuracy become particularly important where the CO content of the gas under investigation may be changing frequently and by significant amounts.
Various types of CO detectors have been proposed and some have been commercialized, but none have provided all of the desired characteristics. Some of the techniques used have been spectrophotometric, spectrochemical, and electrochemical. Another class of detectors, known as solid-state detectors, uses metal oxides which chemisorb CO, resulting in a change of the electronic properties of the metal oxides, which change is measured by detected current or resistance changes.
Another approach, which the present invention utilizes, is oxidation of the CO, accompanied by measurement of the temperature changes caused by oxidation. This has been attempted before, as evidenced by such patents as Lanneau U.S. Pat. No. 3,560,160; Brown et al. U.S. Pat. No. 4,036,592; Jones U.S. Pat. No. 4,072,467; and Jones et al. U.S. Pat. No. 4,123,225.
It appears that the prior art temperature-responsive detectors are deficient in various respects, primarily due to the problem of "poisoning" of the catalysts used to promote the oxidation process. The term "poisoning" refers to the loss of effectiveness of the catalyst material caused by its reaction with CO and/or other gases to which it is exposed. As such poisoning occurs, the reliability of the detector is diminished or destroyed because of reduction in the ability of the catalyst to cause the oxidation of CO. All of the catalysts proposed by the patents cited above are subject to the "poisoning" problem, including hopcalite, manganese oxide, cupric oxide, platinum-rhodium alloys, platinum, and palladium.
In general, the purpose of the present invention is to provide a carbon monoxide detector which will not have the shortcomings of prior art detectors, and will possess the following characteristics: (1) linear response over a wide range of CO concentration, (2) adaptability for either portable or stationary analyzers, (3) cost competitiveness, and (4) relative freedom from most of the common interferences.